Ducts & HVAC systems in the attic are a generally lousy idea, since that pokes big holes in the insulation & pressure boundaries of the house, and puts the ducts in a cold space in winter and a hotter-than-outdoors place during the summer, which would add a lot to the heating & cooling bills (and system size.) If your ceiling heights can take it, running any new ducts in at the ceiling, under the attic insulation is a far better approach.
If the budget is big enough and you can hit at least R50, you might consider insulating at the roof deck, sealing off any roof venting works, in which case putting it all in the attic is fine. You need the R50+ to avoid ice damming issues in an unvented roof design in your area, and it typically involves quite a bit of spray foam insulation (or rigid foam on top of the roof deck) to do it in a way that won't rot out your roof deck over time.
What are your current wall/attic/floor insulation type & values? How about the BTU in/out ratings on the current furnace (and it's fuel type)? House size? How much fuel do you burn through in a heating season?
From a heating point of view you'd be better off with a hydronic (pumped hot water) boiler, which is both more efficient & more comfortable, and doesn't take up a lot of room space, but it's more expensive than ducted hot air. Sizing both the boiler and radiation right is critical, and is typically oversized unless designed by a true professional. Heated floors are the total lap of luxury, but are probably a budget-buster.